This invention relates to numerical control devices, and more particularly to a numerical control device (hereinafter referred to merely as "an NC device", when applicable) which allows the spindle of a machine, which has a spindle head with a plurality of tapping tools, to be fed most suitably with respect to rotation of the tapping tools.
FIG. 6 is a diagram showing the arrangement of a conventional NC device.
In FIG. 6, reference numeral 1 designates a memory comprising a RAM, which stores programs and data; 2, a control section essentially including a CPU 2a, which controls the functions of the NC device according to the system software loaded in the memory 1; 3, an arithmetic operation section for performing numerical operations under the control of the control section; 4, a tape reader for reading a paper tape in which an NC machining program (hereinafter, referred to merely as "a machining program", when applicable) has been stored by perforation; 5, a program interface; 6, a feed shaft drive section including a servo amplifier etc.; 7, a feed shaft motor for driving a feed shaft; 8, a feed shaft encoder for outputting a feedback signal; and 9, an input/output signal interface provided for input signals required for starting and stopping the NC device, and for output signals such as an NC device state signal and auxiliary function signals for controlling external equipments which are controlled by the NC device. The aforementioned machining program may be stored in a floppy disc instead of a paper tape.
Further in FIG. 6, reference numeral 20 designates a speed reducer; 21, a drive mechanism such as a ball screw; and 22, the movable table of the machine on which a workpiece is set.
Further in FIG. 6, reference numeral 26 designates the aforementioned spindle; 27, a spindle motor; 28, a spindle amplifier for amplifying a spindle motor drive signal; 30, a gear train for changing the rotation number (rpm) of the spindle motor 27 to transmit the torque of the latter to a spindle head 31 (described later); 31, the aforementioned spindle head holding a plurality of tapping tools 161 through 164 and transmitting the torque of the spindle motor 27 transmitted through the gear train 30, to the tapping tools; 32, a spindle encoder, or a detector coupled to the spindle motor 27 at a rate of 1:1; and 33, a spindle encoder interface for applying the output of the spindle encoder 32 to the control section 2.
The above-described tapping tools 161, 162, 163 and 164 are different in pitch. Therefore, in tapping a workpiece 15 with the tapping tools, the feed shaft is moved according to the average of the pitches of the tapping tools which is manually calculated so that the load of the tapping tool cushioning mechanism is reduced as much as possible.
The tapping tool cushioning mechanism functions as follows: The tapping tool moves one pitch every revolution (in the case where it is of a single threaded screw). When the movement of the tapping tool is not coincided with the mechanical feed, the difference in movement between both is absorbed by the tapping tool cushioning mechanism.
In the case of FIG. 6, the workpiece 15 is moved. However, it goes without saying that, with the workpiece 15 held stationary, the spindle 26 may be moved by the feed shaft motor 7 while the tapping tools are being rotated.
With the above-described conventional NC device, the average pitch of the plurality of tapping tools is calculated manually, as was described above. This operation takes a lot of time, and will be more troublesome in the case where a workpiece is machined with a machine having a spindle head which is replaceable.